Magnetoresistive (MR) heads are employed in magnetic data storage systems, such as magnetic disc drives, to read data from the storage media. While the MR head is flying over the surface of the magnetic storage media to provide a read-back signal that corresponds to data written on the storage media, it sometimes hits a defect known as an asperity. MR heads exhibit a change in resistance in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the MR element of the head. The direct current (DC) value of the voltage, for a given head, is the product of the constant bias current and the total resistance between the head's lead terminals.
The mechanical collision between the MR head and a defect or asperity can locally increase the temperature of the MR element by more than 100° C. This event has been termed a “thermal asperity”. Since the change in resistance of the MR element, as a function of the magnetic field due to the data stored on the media, is less than 1% of the total MR element or strip resistance, the signal step that is added to the read-back signal when a thermal asperity is encountered can be greater than twice the base-to-peak read signal. For example, an increase in the temperature of the MR element of 100° C. would typically cause a resistance change and a voltage change of approximately 2%. When the protrusion on the disc is persistent, and thus the MR head continues to strike it with each revolution, then the data that is being modulated by the resultant thermally induced signal transient will be unreadable without a sufficient error correction code. Methods and apparatus for categorizing the magnitudes of thermal asperities, and thus the size of the defect on the media surface, would be a significant improvement in the art.
Embodiments of the present invention offer advantages which can be useful in categorizing magnitudes of thermal asperities in data storage systems.